Boundary effect of mechanical waves of a blunt trauma can be exploited for reducing amplitude of the mechanical waves delivered to a brain tissue, using a multi-layered protective shell to increase number of boundaries inside the protective shell of a protective headgear as practically many as possible to a point there would not be a serious tissue injury to the brain tissue. Separately in a model of a two-layer medium panel with a first layer adjoining a second layer without a gap, it is known that there is no phase change at a boundary between the first layer and the second layer having a lower hardness than that of the first layer in reflected mechanical waves from incident mechanical waves traveling from the first layer to the second layer. Combination of both the incident and reflected mechanical waves in phase with each other temporarily increases an amplitude of the incident mechanical waves which increases an amplitude of transmitted mechanical waves in the second layer from the incident mechanical waves. If a series of the incident mechanical waves impacts the first layer, an amplitude of the reflected mechanical waves off the boundary merges with an amplitude of successive mechanical waves following a first wave of the mechanical waves coming toward the first layer. The amplitude of the successive mechanical waves following the first wave of the mechanical waves temporarily increases upon the addition of the amplitude of the reflected mechanical waves in phase with the successive mechanical waves, which increases a magnitude of an impact of the successive mechanical waves following the first wave of the mechanical waves to the second layer. If the first layer is made of a material that has a lower hardness than that of the second layer, the reflected mechanical waves off the boundary between the first and the second layers from the first wave reverse the phase and merge with the successive mechanical waves coming toward the first layer in a way the amplitude of the successive mechanical waves decreases. It results in a reduction of the magnitude of the impact of the successive mechanical waves to the second layer.
To improve on efficiency of the protective headgear in reduction of an amplitude of mechanical waves of the blunt trauma to a human brain, a basic motif of the present invention for the protective headgear comprises an at-least four-layer shell having a first and outermost layer being softer than a second layer which is hardest and made undeformable, an innermost layer being softer than a human skull, and a third layer in between the innermost layer and the undeformable second layer. The third layer in between the innermost layer and the second layer is softer than the innermost layer and the second layer. All three layers except the second layer are to an extent compressible and depressibly deformable by an impact of the blunt trauma at an angle to a planar surface of each layer. Each layer is configured to have a measurable thickness and to be placed next to adjacent layers tightly without a gap.
The incident mechanical waves of the blunt trauma are carried in an incident mechanical three-dimensional columnar force centripetally hitting a single point of maximum impact which produces a maximum divergent, centrifugal reflection off a boundary of the protective headgear. The single point of the maximum impact on the boundary of the protective headgear becomes a single point of entry of a maximum transmitted mechanical three-dimensional force having transmitted mechanical waves toward the brain tissue. Transmission of the transmitted mechanical three-dimensional force having transmitted mechanical waves from the single point of entry focuses the transmitted mechanical three-dimensional force having transmitted mechanical waves on a geographically confined area of the brain tissue, in a way the geographically confined area of the brain tissue would receive a highest amplitude of the transmitted mechanical waves carried in the transmitted mechanical three-dimensional force. One of methods to reduce intensity of the tissue injury to the brain tissue upon the blunt trauma is to focus a reflected mechanical three-dimensional force having reflected mechanical waves out of phase with the incident mechanical waves on a destructive interference with the incident mechanical three-dimensional force having the incident mechanical waves. The incident mechanical three-dimensional force having the incident mechanical waves to the protective headgear in a hemispherical bowl shape can be understood as a liquid jet centripetally hitting a convex contour of the hemispherical bowl, which produces widespread radially scattered, centrifugal reflected liquid streams away from a longitudinal axis of the incident mechanical three-dimensional force. Although the phase of the reflected mechanical waves can be made reversed from a phase of the incident mechanical waves by a lower hardness of the first layer than that of the second layer of the protective headgear, an overall efficiency of the destructive interference with the incident mechanical three-dimensional force having the incident mechanical waves depends on a degree of coaxial convergence of the reflected mechanical three-dimensional force having the reflected mechanical waves with the incident mechanical three-dimensional force having the incident mechanical waves.
The reflected mechanical three-dimensional force having the reflected mechanical waves off the convex contour of the protective headgear can be made coaxially converge with the incident mechanical three-dimensional force having the incident mechanical waves by directing radial axes of reflection of the reflected mechanical three-dimensional force toward the longitudinal axis of the incident mechanical three-dimensional force. A polygon having protruded ridges encircling the polygon disposed on the convex contour of the protective headgear circumferentially surrounding the incident mechanical three-dimensional force to the protective headgear can be configured to produce directed reflections of the reflected mechanical three-dimensional force off the raised borders. A polygonal grid having a plurality of polygons in a configuration of a hemispherical polyhedron can be inserted in between the first layer and the second layer of the protective headgear. A protruding ridge between two adjacent polygons of the hemispherical polyhedron is configured to serve as a point of reflection of the incident mechanical three-dimensional force. A protruding ridge is provided in a configuration of a solid bar. Depending on a cross-sectional configuration of the protruding ridge, such as a rectangular bar configuration or an isosceles trapezoid configuration, angle of the reflection of the reflected mechanical three-dimensional force off the protruding ridge becomes controllable. In terms of the hardness, the polygonal grid is made less hard than the second layer but harder than the first layer. Difference in the hardness of the polygonal grid from that of the first layer induces a destructive interference with the incident mechanical waves by phase-reversed reflected mechanical waves reflecting off a boundary between the first layer and a plurality of the protruding ridges of the polygonal grid.
Intensity of an amplitude of the incident mechanical waves delivered to the brain tissue depends on a mass (weight) of a source generating the incident mechanical waves multiplied by a velocity of an impact from the source and a mass (weight) of a victim and a stopping distance of the impact by the victim colliding with the source: KE=½×mv2 where KE is kinetic energy before an impact, m is mass in kg and v is velocity in meter/second. Since the stopping distance of the impact by the victim is a relatively fixed value (a head does not fall off from a body) and the velocity of the impact from the source could be a relatively fixed value depending on a type of collision, the weight of both the source and victim for the most part would determine the amplitude of the incident mechanical waves from the impact. What this suggests is that an one-size-fits-all protective headgear is not proper for people having a similar size of head but with a range of different body weights. A person with a lighter body weight as a victim of an impact of a blunt trauma to a head will sustain a less powerful amplitude of incident mechanical waves of the impact than a person with a heavier weight. A person with a heavier weight as a victim of a blunt trauma to a head may not be protected well by a multi-layered protective headgear which is made to protect a person with a lighter weight. Since the collision between the source and the victim is a bidirectional process, one of methods to accommodate variable weights of people wearing the multi-layered protective headgear of a similar size is to vary thickness and density of inner layers directly covering the head of a person depending on a body weight of the person. A person with a heavier weight is protected better by a multi-layered protective headgear with thicker and/or denser inner layers.